1. Field of the Invention
The present invention relates to an encoder provided with giant magnetoresistive effect elements that demonstrate very large resistance variation in response to the variation of external magnetic fields.
2. Related Art
A magnetic field sensor using giant magnetoresistive effect elements is disclosed in, for example, the Japanese Published Unexamined Patent Application No. Hei 8-226960, in which four giant magnetoresistive effect elements are electrically connected in a bridge circuit.
As shown in FIG. 16, a magnetic field sensor A disclosed in this application comprises separately located giant magnetoresistive effect elements 1, 2, 3, 4, in which the giant magnetoresistive effect elements 1, 2 are connected by a lead 5, the giant magnetoresistive effect elements 1, 3 are connected by a lead 6, the giant magnetoresistive effect elements 3, 4 are connected by a lead 7, the giant magnetoresistive effect elements 2, 4 are connected by a lead 8, an input terminal 10 is connected to the lead 6, an input terminal 11 is connected to the lead 8, an output terminal 12 is connected to the lead 5, and an output terminal 13 is connected to the lead 7.
And, the giant magnetoresistive effect elements 1, 2, 3, 4 each assume a sandwich structure in which a non-magnetic layer 15 is interpolated between upper and lower ferromagnetic layers 16, 17, and an antiferromagnetic exchange bias layer 18 is formed on the one ferromagnetic layer (pinned magnetic layer) 16, whereby the exchange coupling generated by this exchange bias layer 18 pins the magnetization axis of the ferromagnetic layer 16 in one direction. Further, the orientation of magnetization axis of the ferromagnetic layer (free magnetic layer) 17 on the other side is made to freely rotate in accordance with the orientation of the external magnetic field; for example, it is made to freely rotate on the horizontal plane including the ferromagnetic layer 17.
Further, in the magnetic field sensor A having the structure shown in FIG. 16, the orientation of magnetization axis of the pinned magnetic layer 16 of the giant magnetoresistive effect element 1 faces forward as shown by the arrow 20 in FIG. 16, the orientation of magnetization axis of the pinned ferromagnetic layer 16 of the giant magnetoresistive effect element 2 faces backward as shown by the arrow 21, the orientation of magnetization axis of the pinned magnetic layer 16 of the giant magnetoresistive effect element 3 faces backward as shown by the arrow 23, and the orientation of magnetization axis of the pinned magnetic layer 16 of the giant magnetoresistive effect element 4 faces forward. And, the orientation of magnetization axis of the free magnetic layer 17 of each of the giant magnetoresistive effect elements 1, 2, 3, 4 faces to the right as shown by the arrow 24 in FIG. 17, in the state that the external magnetic field is not exerted.
In the magnetic field sensor A shown in FIG. 16, when an external magnetic field H is exerted, in the first and fourth giant magnetoresistive effect elements 1, 4, for example, the magnetization axis 24 of the free magnetic layer 17 rotates by a specific angle d as shown in FIG. 17, in accordance with the external magnetic field H; accordingly, the relation of angle to the magnetization axis 20 of the pinned magnetic layer 16 varies to effect a resistance variation. And, since the orientations of magnetization axes of the pinned magnetic layers 16 of the first and fourth giant magnetoresistive effect elements 1, 4 face opposite with the difference of 180xc2x0 to the orientations of magnetization axes of the pinned magnetic layers 16 of the second and third giant magnetoresistive effect elements 2, 3, the resistance variation involving a phase difference can be acquired.
In the magnetic field sensor A electrically connected in a bridge circuit shown in FIG. 16, the orientations of magnetization axes are specified as shown by each of the arrows, since the differential output has to be obtained from the giant magnetoresistive effect elements 1, 2, 3, 4 when the orientations of magnetization axes of the free magnetic layers 17 vary in response to the external magnetic field H, and in the giant magnetoresistive effect elements 1, 2, 3, 4 located right and left, upper and lower in FIG. 16, the magnetization axes have to be pinned in antiparallel directions such that any two adjacent elements are magnetized in the opposite directions with 180xc2x0.
In order to achieve the structure shown in FIG. 16, it is imperative to form the giant magnetoresistive effect elements 1, 2, 3, 4 adjacently on a substrate, and fix the orientations of magnetization axes of the pinned magnetic layers 16 of any adjacent two of giant magnetoresistive effect elements opposite each other with the difference of 180xc2x0.
Further, in order to control the orientations of magnetization axes of the pinned magnetic layers 16 of this type, and adjust the lattice magnetization of the exchange bias layer 18, it is imperative to apply a magnetic field of a specific direction to the exchange bias layer 18 while it is heated at a higher temperature than the so-called blocking temperature at which the ferromagnetism disappears, and in addition to conduct a heat treatment to cool while this magnetic field is maintained under application.
However, in the structure shown in FIG. 16, since the orientations of magnetization axes of the exchange bias layers 18 must be shifted by 180xc2x0 to one another for any two of the giant magnetoresistive effect elements 1, 2, 3, 4, the directions of the magnetic fields must be controlled individually for each of the giant magnetoresistive effect elements adjacently formed on a substrate. Since the method of applying a magnetic field simply from outside by using the magnetic field generator such as an electromagnet or the like allows application of the magnetic field only in one direction, it is very difficult to fabricate the structure shown in FIG. 16, which is a problem.
The technique disclosed in the Japanese Published Unexamined Patent Application No. Hei 8-226960 indicates that the structure shown in FIG. 16 can be achieved by depositing conductive layers individually along each of the giant magnetoresistive effect elements 1, 2, 3, 4 adjacently formed on a substrate, and conducting the foregoing heat treatment by flowing currents in each of these conductive layers in different directions to individually generate magnetic fields of different directions from each of the conductive layers. However, even if it is desired to generate high magnetic fields by applying high currents to the conductive films in order to control the lattice magnetization of the exchange bias layers 18, it is difficult to flow high currents through the thin conductive films that are deposited with the giant magnetoresistive effect elements on the substrate, and difficult to generate the magnetic fields from the conductive films, which are sufficient for the subsequent processes. Further, since the magnetic fields are exerted on the giant magnetoresistive effect elements 1, 2, 3, 4 adjacently formed on a substrate, in each different directions from a plurality of the conductive films, it is extremely difficult to individually apply the high magnetic fields to each of the exchange bias layers 18 of the giant magnetoresistive effect elements 1, 2, 3, 4.
As mentioned above, the magnetic field sensor A shown in FIG. 16 possesses an excellent function as a magnetic sensor; however in reality, to form the films on a substrate and fabricate the magnetic field sensor A involves extremely delicate processes to apply the magnetic fields and heat processes, making the fabrication difficult, and the structure causes a problem for a wider applications.
Further, as to the applications of the magnetic field sensor A shown in FIG. 16, the Japanese Published Unexamined Patent Application No. Hei 8-226960 only suggests the applications to linear and rotary encoders, proximity sensors, geomagnetic magnetometers, and the like. And, there are not any concrete suggestions as to which equipment and fields the structure of the magnetic field sensor A is to be applied to.
On the other hand, as an example of products applying the magnetism, a magnetic encoder is well known. The encoder of this type uses the Hall elements as the detecting elements responsive to the variation of the magnetic field, however the output waveform before processing, namely the waveform generated by the Hall elements, is approximately the sine curve. A drift of unbalanced voltage of the Hall elements, or a drift of input offset voltage of an amplifier, deteriorates both the duty (ratio between low and high) of a rectangular wave obtained, and the phase difference between A and B phases, which are the problems. In addition, the output waveform generated from the Hall elements is low and easy to be influenced by these factors. Accordingly, the use of the Hall element has made it difficult to acquire a highly precise signal. Therefore, it has been eagerly sought to achieve an encoder that possesses detecting elements to generate a high output with a waveform approximate to the rectangular wave and generates a highly precise signal.
In view of the foregoing circumstances, the present invention has been made through trials to employ the giant magnetoresistive effect elements, which is based on a novel idea unlike the conventional magnetic field sensor. It is therefore an object of the present invention to provide an encoder that detects the angle of rotation of a magnetic coding member and obtains a higher output by adopting a unique structure using the giant magnetoresistive effect elements.
In order to accomplish the foregoing objects, the encoder of the present invention comprises at least a pair of giant magnetoresistive effect elements that contain at least exchange bias layers, pinned magnetic layers whose orientations of magnetization axes are fixed in one direction by the exchange bias layers, non-magnetic layers, and free magnetic layers whose orientations of magnetization axes are freely rotated by an external magnetic field. And, the giant magnetoresistive effect elements to be paired are formed on a substrate in a state that the elements are connected mutually electrically with the orientations of magnetization axes of the pinned magnetic layers each facing the same direction in parallel, and a magnetic coding member is rotatably supported to face the giant magnetoresistive effect elements on the substrate, and the magnetic coding member is provided with a plurality of magnetic poles formed along the direction of rotation of itself.
The encoder of the present invention may take on a construction such that an output terminal is formed on a part that connects one end of one of the giant magnetoresistive effect elements to be paired to one end of another, and input terminals are each formed on the other end of one giant magnetoresistive effect element and on the other end of the other giant magnetoresistive effect element.
The encoder of the present invention may take on another construction such that the magnetic coding member is formed in a disc-shape, a plurality of different magnetic poles are alternately formed with a specific pitch on the periphery of the magnetic coding member, and the magnetic coding member is supported to freely rotate with a specific gap detached from the substrate, in a state that the periphery of the magnetic coding member faces the giant magnetoresistive effect elements.
The encoder of the present invention preferably has a construction such that a gap between the giant magnetoresistive effect elements to be paired has a relation that satisfies an expression of 2nxcex+xcex, when the polarization pitch on the magnetic coding member is represented by xcex and the integer by n.
Further, the encoder of the present invention preferably has a construction such that a first, a second, a third, and a fourth giant magnetoresistive effect elements are formed in a line on the substrate in parallel to each other with a specific gap detached from each other, and the gap between the giant magnetoresistive effect elements has a relation that satisfies the expression of 2nxcex+xcex, when the polarization pitch (distance between the N pole and S pole) on the magnetic coding member is represented by xcex and the integer by n.
Further, the encoder of the present invention may take on another construction such that, of parts to connect the first, second, third, and fourth giant magnetoresistive effect elements, two parts have input terminals formed thereon, and the remaining two have output terminals formed thereon.